The attenuation correction is one of the most important steps in emission tomography imaging to assure a good image quality. In particular, attenuation correction is important for single-photon emission computed tomography (SPECT). Attenuation occurs when an emitted photon interacts with a material, such as patient tissues. In general, such attenuation due to several types of interactions with the patient tissues. For example, a photo-electric event can occur in which the photon is effectively stopped by the material. In another example, Compton scatter of the photon can occur, in which the photon is deflected from its original path, but with a loss of energy.
As a result of attenuation, a conventional SPECT system will generally detect less photons from deeper structures in the body than from tissues near the body surface. For example, in reconstruction of brain scans, there will generally be lower counts from the deeper structures in the brain (e.g., the basal ganglia).
One methodology for providing attenuation correction is to utilize a transmission-based imaging technique during SPECT imaging. For example, attenuation correction for SPECT is traditionally provided using data from an X-ray computed tomography (CT) scan concurrently obtained during SPECT imaging. However, such a methodology has several issues. First, many existing SPECT imaging systems do not include CT scan equipment and thus the necessary CT images cannot be obtained. Second, if attenuation needs to be performed on older image data, it is not possible to obtain the necessary CT scan data afterwards. Third, there may be concerns regarding dosing individuals with X-ray radiation. For example, in the case of pediatric patients, there is a general desire to limit exposure to X-ray radiation.
However, since the tissue structures of the brain can be considered to be generally uniform, the attenuation for the brain can also be generally considered to be constant. In the case of such uniform structures, the method described by Chang in “A method for attenuation correction in radionuclide computed tomography”, IEEE Trans Nucl Sci 1978; 25:638-643, can then be applied without the need for a CT scan or other transmission-based imaging. Accordingly, many commercial SPECT systems are therefore configured to allow use of the filtered back projection (FBP) with the method of Chang to provide attenuation correction. The use of FPB with the method of Chang is an approximate method, which simply calculates the average attenuation for photons travelling from each point in the body at different angles. In general the method of Chang consists of performing a multiplication by a correction factor at each point, with typically slight over-correction, even when using an effective attenuation coefficient.
However, the method of Chang has several deficiencies. First, operators must ensure that the correct body boundary is selected for the part of the body being corrected, since the algorithm will assume a constant attenuation within that boundary. For example, the required boundary is the edge of the head, not the edge of the brain, and this boundary is different in each transverse slice through the head. Second, the FBP method of Chang requires a user to manually specify the mu-values for different materials. Third, FBP methods, such as those of Chang, are known to provide inferior image quality as compared to a 3D iterative reconstruction methods used in other modern molecular imaging.